1. Field of the Invention
The present invention generally relates to a multilayer wiring board having a transmission line structure of high wiring density, and in particular to a conductor-embedded type of a multi-layer wiring board having transmission line structure composed of at least a pair of adjacent signal conductors embedded near a surface of a dielectric layer and also relates to a manufacturing method thereof.
2. Description of the Prior Art
FIG. 8 shows a wiring pattern model of a conventional multi-layer wiring board or multi-chip waveguide generally used in information equipment and the like. In this construction, a characteristic impedance representing a signal transmission characteristic of line conductors or wires for connecting semiconductor components and the like elements is defined as Z0={square root over (L/C)} where C is a capacity and L is an inductance per a unit length of the line conductor or wire. In this definition, the capacity C and inductance L are determined by, e.g., a width and thickness of the conductor of the wiring pattern and a thickness of an insulating layer region, and therefore the characteristic impedance Z0 is also determined by these factors.
In designing a wiring structure, the value of the characteristic impedance Z0 is determined in consideration of a higher-speed processing, circuit system, crosstalk value and the like of semiconductor components, and the value of the characteristic impedance of the wiring structure for use in information equipment is generally in a range of several tens xcexa9 to one hundred xcexa9. Unification or equality of characteristic impedance values is also especially important in view of reducing noises. If there exists a discontinuous point in a wiring structure, a reflection occurs to cause an erroneous operation in the information equipment.
The following explains a typical example of a transmission waveguide structure of a printed wiring board having an insulation region composed of a dielectric layer. FIG. 9 shows a cross section of a basic form of a microstrip waveguide, where a strip signal conductor 3 is formed on one surface (upper surface in the drawing) of the dielectric body (i.e., board or layer) 1 and a flat plane conductor 2 such as a grounding conductor layer is formed on the other surface (bottom surface in the drawing) of the dielectric board 1 so as to generate an electric field (electric power lines) across the center portion and edge portion of the dielectric body as shown by dotted lines in the drawing.
The thickness of the dielectric board 1 is T and a signal conductor 3 having a width w and thickness (i.e., height) t is formed on the upper surface thereof in the drawing. For example, approximate values thereof are used as such that, the thickness T is 220 xcexcm, width w is 360 xcexcm and thickness t is at least 10 xcexcm. Thus, the characteristic impedance of the signal conductor 3 with respect to the lower plane conductor 2 is approximately determined by a dielectric constant ∈ of the dielectric material and thickness T of the dielectric board 1 and the width w of the signal conductor 3 disregarding the thickness t of the signal conductor 3 because the thickness t of the signal conductor 3 is enough small with respect to the width w (i.e., t less than  less than w). Therefore, difference or unevenness in characteristic impedances is caused due to differences in width w and thickness T, and because of the small thickness t of the signal conductor 3, there arises a problem that a loss increases in electric consumption as a resistance of the signal conductor increases when constructing a wiring pattern with high wiring density.
FIG. 10 shows a wiring board having a plurality of line conductors 3a through 3e. Assuming that each gap distance between adjacent two conductors is substantially equal to the width of each signal conductor 3, three usage examples thereof will be explained as below.
In the first example, the respective conductors are used as independent five signal conductors. In the case of a wiring structure having a characteristic impedance of around 50 xcexa9 which is normally used, a coupling impedance (also regarded as a coupling capacity) between the adjacent signal conductors is fairly higher in dimension than the coupling impedance between the signal conductors and the plane conductor 2. However, a cross-talk between the adjacent signal conductors becomes a problem.
In the second example, a shielding conductor is placed between signal conductors in order to reduce a cross-talk. For example, the line conductors 3b and 3d are used as signal conductors, and the line conductors 3a, 3c and 3e are used as the shielding conductors having the same potential as that of the plane conductor 2. Generally, in order to lower the cross-talk, in comparison with the case where gaps between the signals conductors are increased, the provision of the shielding conductors makes the wiring density of the transmission line structure to be higher.
In the third example, the adjacent two line conductors 3b and 3c are used as a pair of balanced transmission line conductors, and the line conductors 3a and 3d positioned at both sides thereof are used as the shielding conductors.
FIG. 11 shows another example of a conventional microstrip waveguide structure in which the dielectric region 1 and plane conductors 2 are provided over both upper and lower surfaces of the signal conductor 3. In this structure, in the case where the microstrip waveguide is incorporated in a multi-layer wiring board, interference with other wiring layers can be reduced, and excellent transmission characteristics can be obtained.
FIG. 12 shows an example of a conventional coplanar transmission waveguide structure, where a signal conductor 3 and a pair of plane conductors 4 are provided on the same surface of the dielectric board 1. This coplanar type can be composed of one wiring layer, but high density of the transmission lines cannot be obtained, and the coplanar type is easily influenced by surroundings or conductors on another layer. For this reason, this coplanar transmission waveguide structure is not suitable to be incorporated into a multi-layer wiring board.
The aforementioned conventional transmission waveguides are manufactured in such a manner that a signal wiring pattern is formed by photolithographing and etching a thin copper foil which is stuck to the dielectric board 1. Also, in a conventional planar type of a transmission waveguide structure, it is difficult to make an aspect ratio of each line conductor larger than 1 even by any method of a plating, printing or etching.
As a signal processing ability such as a microprocessor has been improved by a high-integrating technique of semiconductor IC year by year, a wiring board provided with such a microprocessor requires a severe condition for improvement of wiring density and transmission characteristics as following.
The first problem is to improve a wiring density. When a number of connection terminals of a semiconductor chip and a package is increased, a higher wiring density on the wiring board is required. The wiring density for one wiring board can be improved by multilayering, but the multilayering causes a high cost due to an increase in a number of layers and an increase in an area for via wiring. For this reason, the improvement in the wiring density for one layer is essentially required.
The second problem is to improve transmission characteristics of a transmission waveguide structure. A signal bus construction is particularly important in wiring, and an influence of a noise such as a reflection and a cross-talk becomes large due to a higher speed of a signal rate. Therefore, improvement of a wiring structure is required, reducing unevenness in characteristic impedance of the transmission line conductors when manufacturing the wiring board together with lowering a cross-talk.
In order to increase a wiring density of transmission lines while maintaining a characteristic impedance value constant, the thickness (T) of the dielectric board and the width (w) of the signal conductor should be reduced proportionally. In order to maintain a manufacturing accuracy of a width of a signal conductor by etching, the thickness (t) thereof must be also reduced proportionally. In general, as they are reduced, the control of unevenness in manufacturing becomes more difficult. Moreover, since a cross section of the signal conductor is reduced by the second power of a reducing ratio, there arises a problem of deterioration in transmission characteristics due to increase of a resistance value. In the conventional structure of the transmission waveguide, it is difficult to satisfy the requirement of the first and second problems simultaneously.
It is an object of the present invention to provide a multilayer wiring board having a transmission line structure which enables improvement of a wiring density and a transmission characteristic and to provide a manufacturing method thereof.
In order to solve this problem, a multi-layer wiring board has transmission line structure comprised of at least a pair of adjacent signal conductors which are embedded in a first region near one surface of a dielectric layer, wherein a thickness in height of each of the signal conductors is larger than a width thereof, and the thickness of each of the signal conductors is larger than a gap distance between the adjacent signal conductors.
In this construction, a coupling impedance between the adjacent two signal conductors is lower than a coupling impedance between each of the signal conductors and a conductor formed in a second region near the opposite surface of the dielectric layer.
Moreover, a manufacturing method of a multi-layer wiring board comprises the steps of: providing a conductor in a first region near one surface of a dielectric layer to form transmission lines comprised of at least a pair of adjacent signal conductors which are embedded in the first region; and defining a configuration of the signal conductors so that a thickness in height of each of the signal conductors is larger than a width thereof, and the thickness of each of the signal conductors is larger than a gap distance between the adjacent signal conductors.
In this method, a groove is formed for embedding the conductor in the dielectric layer by molding die for forming the transmission lines.
By this arrangement, the transmission line conductors are embedded in the dielectric layer and each line conductor has an aspect ratio being larger than 1. In this construction, a pitch between the transmission lines is small, and the wiring density is made high, and further exhibiting small wiring resistance considering the width of the signal conductor and favorable matching properties of characteristics impedance of the transmission lines. It is thus possible to achieve both, high density wiring as well as high-speed and excellent transmission characteristics.